Update eav.html

Extending model_to in related models
Correcting small typo errors
Merge pull request #613 from kay899/1.8/develop

Update eav.html
Update eav.html

Correcting typo errors
Update usage.html

https://github.com/fuel/docs/issues/615
Update db.html
Merge pull request #616 from EmRa228/patch-1

Update usage.html
Merge pull request #617 from EmRa228/patch-2

Update db.html
change vales to values
Merge pull request #618 from EmRa228/patch-3

change vales to values
Merge pull request #614 from kay899/1.8/develop

Update eav.html
本家変更分をmerge